On 4/05/2023 03:55, Andrey Semashev wrote:

The fact that the initial message is not part of the thread now is a relatively recent issue. This is the mailman archive from a year ago:

https://lists.boost.org/Archives/boost/2022/04/index.php

Threading is fine there. In fact, threading was working correctly as recently as in February 2023:

https://lists.boost.org/Archives/boost/2023/02/index.php

and has been broken in March:

https://lists.boost.org/Archives/boost/2023/03/index.php

If the system works (and worked) the way you describe, mailman was able to associate the initial message with the thread before and now it isn't. I suspect, this may have something to do with the headers that are added by SES to the messages it sends to the subscribers.

Threading only sometimes works in the gmane view as well. Usually it's not the mailing list itself at fault, but due to the mail client used by the person posting -- or sometimes that they're replying to a digest rather than the original mail, so the context headers are lost (even if they edit it to look like it was a reply to a single post). I have noticed an apparent increase recently in the number of posts that appear "flat" or semi-flat (still showing as a reply, but to a grandparent post rather than the real parent post) -- but other recent threads still do appear normally nested, so I don't think it's a systemic issue. This is what I'm seeing -- note that the current thread appears semi-flat (even though most of these sibling messages were actually a reply chain), but the "replacing the ML with a forum" thread appears mostly normal. https://imgur.com/a/GfTS6ig